Currently developed high strength steel for use in structural members of automobiles, etc. has a little formability and thus, is difficult to be used in the manufacture of elements having a complex shape.
Accordingly, manufacturers of automobiles have attempted to simplify the shape of elements, or to divide a relatively complex element into several sub-elements, for easy forming of the element.
However, the use of the several divided elements have a need for a secondary welding process. Moreover, since the strength of a welded joint differs from the strength of a base material, there is a serious limit in the design of an automobile body.
For this reason, manufacturers of automobiles have sought continuously for a high-strength steel material with superior formability, so as to use the steel material in the manufacture of elements having a complex shape and to increase a freedom in the designing of an automobile body. Meanwhile, even if the steel material has superior formability and high strength suitable for use in the manufacture of structural members of automobiles, etc., the steel material has a difficulty in a hot-dip galvanizing process if a great amount of an alloying element, more particularly, silicon (Si), is added into the steel material.
Furthermore, in the case where the steel material containing a great amount of silicon is manufactured in a continuous annealing or continuous hot-dip galvanizing line, there is the problem that metal grains in a surface of a steel sheet are dropped out and attached to and stacked on a hearth roll within a continuous annealing facility, thereby causing a dent defect in the subsequent coil.
Therefore, the present invention has been made in view of the above problems, and it is an aspect of the present invention to provide a method for manufacturing a steel sheet having high strength and formability as well as superior hot dip galvanizing properties by appropriately controlling the composition of steel and manufacturing conditions.